1. Field
This disclosure relates to pressure sensors. More particularly, the present disclosure describes microfabricated implantable pressure sensors for use in biomedical applications and methods for such pressure sensing and the resulting systems. Such systems, sensors and methods may be used for monitoring intraocular pressure.
2. Description of Related Art
Pressure sensing is a powerful approach to study physiological conditions in biomedical applications and human healthcare. Glaucoma, hydrocephalus, aneurysms, and other medical conditions can be effectively studied and possibly controlled by monitoring pressure fluctuations. Passive telemetry has been a widely used technique to measure physiological parameters since first reported as far back as 1957. Passive telemetry has advantages such as high measurement accuracy, high precision and wireless operation, which may not require embedded power supplies. Passive telemetry may be one of the viable methods to accomplish continuous and faithful non-contact pressure measurements in biomedical applications, such as intraocular pressure (IOP) sensing/recording for glaucoma patients and blood pressure monitoring for patients with abdominal aortic aneurysms (AAA).
As indicated above, IOP sensing may be used with glaucoma patients. Glaucoma is a debilitating eye disease that chronically damages the optic nerve and results in loss of vision for tens of millions of people worldwide. The disease is associated with abnormally accumulated intraocular fluid and resulting elevated intraocular pressure (IOP). Accordingly, successful IOP monitoring is crucial in the management of glaucoma patients as it is known as one of the most effective methods to evaluate the progression of this eye disease. Current clinical diagnosis involves contact or non-contact applanation tonometry for IOP recording. However, both modalities have difficulties in providing reliable and repeatable measurements and, particularly, in deployment for regular (e.g., daily) tracking, which impedes prompt detection and appropriate treatment for IOP spikes from its diurnal fluctuations considered as a separate risk factor to optic nerve damage. Continuous IOP monitoring in glaucoma patients with high accuracy and high reliability is therefore a consistent need for ophthalmologists.
As noted above, telemetric sensing is one of the viable methods to accomplish continuous and faithful non-contact IOP measurements. It utilizes a transensor implant that registers environmental pressure variations inside the eye, so that the IOP can be directly measured by using an external reader wirelessly interrogating the implant. This methodology enables straightforward IOP sensing without involving further calculations which are derived from ocular mechanics such as used in applanation tonometry and which have large variation due to different dimensions and mechanical properties of individual eyes. In contrast to active sensing in which power transfer, size, and cost of the device are critical concerns, passive sensing approaches have relatively flexible design considerations on the device side.
Miniaturized LC sensors (sensors with electrical LC resonant circuit) have been shown to be feasible for passive wireless pressure sensing for various applications, including transcutaneous pressure monitoring, intracranial pressure monitoring, cerebrospinal fluid (CSF) pressure monitoring, and pressure monitoring of abdominal aortic aneurysms in addition to the proposed IOP monitoring discussed above. Such devices show the use of wireless passive pressure transensors for continuous measurement of physiological parameters in biomedical systems and human healthcare. An example of a passive sensor for pressure sensing using an electrical LC-tank resonant circuit in combination with a corresponding sensing scheme is shown in FIG. 1. In the resonant circuit, a pressure-sensitive capacitor Cs causes associated resonant frequency shifts due to bioenvironmental pressure variations that can be detected by an external reader through electromagnetic coupling between coils Ls inside the body and coils Lr outside the body. Such sensors may be implanted into the site of interest and passively send signals through communications with external readout circuits.
The sensing scheme illustrated in FIG. 1 requires an external reader to interrogate pressure variations electrically registered by an implanted sensor through a wireless inductive coupling link. Accordingly, a relatively long sensing distance between sensor and reader coils in reasonable arrangements is crucial for practical measurements. That is, for example, a sensing system that requires the coils of the external reader to be placed right against an eye for IOP measurements would be of limited usefulness due to the desire to take IOP measurements over an extended period of time. Systems known in the art generally trade off the distance of the external reader coils from the embedded sensor with the resolution and reliability of the measurements. Such systems show a decrease in reliability and resolution as the external reader coils move further from an implanted sensor.
Hence, there is a need in the art that will allow high resolution and reliable pressure measurements to be made with an implanted sensor while the external reader inductively coupled to the implanted sensor is positioned at a distance that allows such measurements to be practicably made over an extended period of time.